This invention relates to a receive antenna selection method and a diversity receive, and more particularly to a receive antenna change-over method and a diversity receiver in a radio packet communication system.
A spread spectrum (hereinafter also referred to as "SS") modulation system is adapted to transmit a signal which has been subject to modulation while substantially increasing a band width of the signal, as compared with that by a narrow-band modulation. The SS modulation systems include a direct sequence (hereinafter also referred to as "DS") system, a frequency hopping (hereinafter also referred to as "FH") system and the like. The spread spectrum system provides satisfactory crosstalk-proof characteristics and exhibits increased resistance to each of jamming, interference by a narrow band and a variation in transmission path. Thus, it has recently come to notice in the field of radio communication such as radio LAN, mobile communication or the like.
In radio communication, radio waves outputted from a transmitter are guided to an antenna of a receiver through a plurality of paths while being reflected by a wall, a ceiling and the like. The radio waves thus propagated through such plural paths interfere with each other at an end of the receive antenna, to thereby amplify or attenuate each other, leading to a phenomenon called multipath fading.
In such a multipath fading environment, even if the SS system which is said to exhibit increased resistance to interference or radio interference, an bit error often occurs due to the fading when most or all of spectra spread falls or a spread code series length is reduced.
In order to eliminate such an influence of the multipath fading, a change-over diversity system has been employed irrespective of the narrow-band communication or SS communication. The change-over diversity systems include (1) a space diversity system constructed so as to use a plurality of antennas arranged at places spaced from each other by a distance corresponding to a quarter wavelength, (2) a polarization diversity system using a horizontally polarized wave and a vertically polarized wave, (3) a frequency diversity system adapted to transmit identical information by means of a plurality of carriers, (4) an angular or directional diversity system having a plurality of directional antennas arranged in directions different from each other, (5) a time diversity system for transmitting identical information in a slot plural times, and the like. In general, a diversity system such as any one of the systems (1) to (3) wherein a plurality of diversity receive antennas are arranged and a receiver selects any one of the antennas which is ready for receiving of a signal is employed.
Changing-over among the plural receive antennas in the diversity system described above is carried out by detecting a failure in receiving of a signal such as, for example, generation of a bit error in a signal received.
Thus, a radio data communication system such as a radio LAN or the like fails in normal receiving of a signal, because even when change-over among the receive antennas is carried out after generation of a bit error, bit data received before the changing-over constitute an error. Employment of the antenna diversity system in the radio data communication system still fails to substantially improve a receive bit error rate.
A radio packet communication system is so constructed that a whole packet carries out error check, to thereby nullify or discard the whole packet when any bit error is detected in even only one bit of the packet. Thus, a bit error in even only one bit affects the whole packet, resulting in transmission efficiency thereof being highly deteriorated.